Affirmative consent was first articulated on college campuses in the United States in the 1990s. Affirmative consent demanded verbal, positive consent during each sexual encounter, and for each individual part of the sexual activity. Consent had to be meaningful and enthusiastic. While later interpretations would shift and change, and in many instances, non-verbal communication might count as affirmative consent, the model required active and positive participation from everyone during a sexual encounter. An absence of agreement—through silence or passivity—does not meet the standard of affirmative consent.This chapter outlines what affirmative consent was and is, how and why it developed, and why it has been widely imagined as a useful tool for navigating sexual encounters, and for preventing sexual violence. It charts what was initially a radical, alternative suggestion, to a concept that is now institutionalised in the mainstream. Just as importantly, it investigates the challenges of affirmative consent, in particular noting that power imbalances do not allow all people the freedom and bodily autonomy to assert consent. The chapter examines cases where affirmative consent cannot be freely given or denied, including situations where intimate partner violence occurs, or where other practices of coercion or powerlessness are in place. Finally, the chapter traces some of the recent legislative changes across Western cultures, which have enshrined affirmative consent in the law, despite the challenges of ensuring equal access to autonomy.